1. Field of the Invention
The present invention relates to a rotary member driving apparatus which controls a rotary member such that the rotary member rotates at a target angular velocity.
2. Description of the Related Art
In copy machines and printers which form an image using a photosensitive drum, there is a problem that an angular velocity of the photosensitive drum varies due to an eccentricity or the like of a mechanism which transmits a driving force to the photosensitive drum. The variation in the angular velocity of the photosensitive drum causes variation in the pitch of a laser beam and color misregistration, and therefore it is difficult to improve the image quality. Thus, to improve the image quality, it is necessary to reduce the variation in the angular velocity.
According to a known technique, the angular velocity of the photosensitive drum is detected with an encoder or the like and feed-back control of a DC motor which drives the photosensitive drum is performed so as to reduce the variation in the rotational driving process by adjusting the driving force in accordance with the detected angular velocity.
According to this method, although variation in the angular velocity at a relatively low frequency can be reduced, variation in the angular velocity at a relatively high frequency cannot be reduced.
Therefore, in addition to the main driving member (DC motor) which drives the photosensitive drum, an auxiliary driving member, such as another motor or a brake, may be additionally used to reduce the variation in the angular velocity. In such a case, variation in the angular velocity with a relatively high frequency (100 Hz to 200 Hz) can also be reduced (see, for example, Japanese Patent Laid-Open No. 2000-330420). On the other hand, in an image forming apparatus including an acoustic wave motor with high rotational accuracy for driving a photosensitive drum, there may be a case in which the photosensitive drum cannot be driven when the torque is generated only by the acoustic wave motor. In such a case, an additional motor may be used to assist the rotation of the photosensitive drum (see, for example, Japanese Patent Laid-Open No. 11-073065).
However, since the auxiliary driving member additionally applies a driving force to the photosensitive drum while the main driving member is controlled so as to rotate the photosensitive drum at a predetermined angular velocity, feed-back control of the main driving member will be affected. Therefore, a complex control system is necessary and there is a possibility that the stability of the control system will be reduced. In addition, in the structure in which a plurality of photosensitive drums are driven by a single drive source, the main driving member and the auxiliary driving member influence each other. Therefore, the complexity of the control system increases and there is a possibility that the stability of the control system will be further reduced. In addition, although it is described in Japanese Patent Laid-Open No. 11-073065 that the additional motor for driving the photosensitive drum is provided with a torque limiter, the velocity control of the acoustic wave motor and the additional motor is not specifically described.